Operators of combustion devices such as boilers have become increasingly aware that continuous detection and measurement of gases produced in minor quantities such as carbon monoxide, and responsive control of the processes which produce them, can result in dramatically improved fuel efficiency. For example, the provision of excess air was formerly widely used in combustion processes on the assumption that a lean mixture would assure more complete combustion of the fuel. However, as combustion processes became better understood, it also became apparent that the use of excess air was wasteful, because among other things it required the flame to heat excess gas, enabled the formation of SO.sub.3 instead of merely SO.sub.2, encouraged the formation of NO, created sulfate emmissions, and in some cases even increased smoke formation by shortening the flame length. Combustion operations using low excess air improve all of the above situations, but the control must be accurate, and be quickly responsive in order to insure complete combustion while avoiding uneconomical operations and the formation of excessive pollutants. The concentration of carbon monoxide produced by a combustion process turns out to be a good measure of the average combustion quality, i.e., nearness to stoichiometric condition. No CO means too much air, while high CO means not enough air.
With the realization that controls based on the concentration of some minor component of a gas stream can lead to an optimized combustion function, serious development of suitable instrumentation was undertaken, especially instrumentation for measuring the concentration of carbon monoxide in a gas stream. Of course, measuring techniques and instruments had long existed for this purpose, but frequently they relied on sampling techniques which were too slow to provide useful data for on-line adjustment of combustion parameters, or not reliable enough for continuous duty.
The increased stringency of government regulations relating to power plant emissions has long been a prod for the development of in-situ gas analyzers, and several types of such analyzers have been installed in hundreds of power plants in recent years. Some utilize the technique known as "gas filter correlation", which is a technique utilized in the instant invention. It is an object of this invention to employ this technique to better advantage in a gas analyzer whose sampling is done "in-situ", meaning without removal of a sample from the stream, but instead securing data as the consequence of measurements or observations of spectral energy which has been subjected to interaction with the gas stream itself--either by having passed through the gas stream or by having emanated from it.
Gas filter correlation is a well-known procedure which does not require description here for an understanding of the invention. A useful reference on this subject is "Analytical Methods Applied to Air Pollution Measurements" by Stevens and Herget, Chapter 10, pages 193-231, published by Ann Arbor Science, 1974, which is incorporated by reference herein for its showing of the applicable theory.
It is an object of this invention to provide a gas analyzer which can have a direct zero and span measurement, even with process sample gas continuing to flow through the process; which can readily and automatically be calibrated, and all interferences automatically rejected; which can be constructed so as readily to be accessed for routine repair and maintenance, and even disposed at a considerable distance from the stack; which is sufficiently heat resistant that its readings do not stray during temperature excursions; which rejects spurious signals from its surroundings; and which is forgiving of substantial physical shifts and changes in the physical environment, such as by expansion and contraction.
Still further objects are to provide better techniques for internal calibration of the instrument, for more efficient optical path, and for decreased sensitivity to external physical distortions such as vibratory and temperature induced dimensional shifts, as well as to interferences from changing gas stream chemical composition.